1. Field of the Invention
The present invention relates to a compact hydrostatic transmission apparatus for a working vehicle with a center differential gearing.
2. Background Art
There is a well-known conventional transmission apparatus for a working vehicle, which distributes power of an engine between a pair of front wheels and a pair of rear wheels, which are differentially connected to each other through a center differential gearing, so as to enable a vehicle having the transmission apparatus to smoothly turn. Japanese Utility Model Application Laid Open Gazette No. Hei 1-72416 discloses a conventional arrangement of a center differential gearing in a transmission apparatus having a hydrostatic transmission (hereinafter referred to as an xe2x80x9cHSTxe2x80x9d), wherein the center differential gearing is interposed between an output shaft for driving front wheels and an output shaft (a final pinion shaft) for driving rear wheels, which are arranged coaxially with each other. Between a motor shaft of the HST and the output shaft for driving front wheels extended forward from the center differential gearing, is interposed a gearing for speed change having a counter shaft disposed in parallel thereto.
Japanese Patent Application Laid Open Gazette No. Hei 4-297334 also discloses a center differential gearing having a cylindrical differential casing, wherein a gearing for speed change is interposed between the differential casing and a shaft that is parallel to the differential casing, and an output shaft for driving front wheels and an output shaft for driving rear wheels disposed coaxially with each other. These shafts are inserted into the differential casing and differentially connected to each other therein.
Thus, a transmission casing is required to be expanded vertically or laterally for housing the conventional center differential gearing constructed on the assumption that the gearing for speed change is interposed between an input member (usually, a differential casing) of the center differential gearing and a shaft which is disposed in parallel to the input member and on the transmittal upstream side of the input member. In other words, the conventional center differential gearing hinders minimization of a transmission apparatus having an HST.
An object of the present invention is to provide a compact transmission apparatus for a working vehicle, including an HST, which outputs power to sun-and-planetary gearing which differentially connects rear wheels to front wheels.
To achieve the object, a transmission apparatus for a working vehicle according to the present invention comprises an HST including a hydraulic pump which is driven by a prime mover equipped on the working vehicle, a hydraulic motor fluidly connected to the hydraulic pump, a hollow motor shaft serving as an output shaft of the hydraulic motor, and sun-and-planetary gearing including first, second and third elements arranged coaxially to the motor shaft. The first element is drivingly connected to the motor shaft so as to be rotated integrally with the motor shaft. The second element is drivingly connected to front wheels of the working vehicle, and the third element is drivingly connected to rear wheels of the working vehicle and differentially connected with the second element through the first element.
That is, in the sun-and-planetary gearing serving as a center differential gearing, the first element, e.g., a differential casing and a carrier, rotated integrally with the motor shaft, the second element, e.g., a front wheel driving shaft, interlocking with the front wheels, and the third element, e.g., a rear wheel driving shaft, interlocking with the rear wheels, are arranged coaxially to the motor shaft of the HST so as to receive rotation of the motor shaft not through the counter shaft. This saves parts and costs and realizes a compacting transmission apparatus in which a transmission casing is especially minimized so as to ensure a sufficient ground clearance for a mower disposed below the transmission casing. Moreover, since the motor shaft is disposed co-axially with the sun-and-planetary gearing so that the driving force from the motor may be directly transmitted to the sun-and-planetary gearing, the capacity of the HST corresponding to the torque for power transmission to the sun-and-planetary gearing is allowed to be set small.
Preferably, in the transmission apparatus, a differential locking mechanism for locking the second and third elements with each other is provided for equalizing circumferential speed between the front wheels and the rear wheels. Since the second and third elements in the sun-and-planetary gearing are mutually and integrally rotated by the differential locking mechanism, the transmission apparatus enables a vehicle to stably travel on soft or rough ground while preventing the front and rear wheels from idling. This improves the driving performance of the vehicle.
Further preferably, at least one of the second and third elements includes a shaft drivingly connected to either the front or rear wheels, and the differential locking mechanism is disposed on the shaft. In the sun-and-planetary gearing, each of the second and third elements essentially includes such a shaft drivingly connected to either the front or rear wheels in addition to a gear. Therefore, the space around this shaft is used for arranging the differential locking mechanism so as to keep the compact layout in the transmission casing.
With respect to the relative position of the sun-and-planetary gearing and the HST, the sun-and-planetary gearing is preferably disposed forward or rearward of the HST. Further preferably, the transmission apparatus includes a housing for the HST while the sun-and-planetary gearing is disposed outside the housing. Further preferably, a second housing for incorporating the sun-and-planetary gearing disposed outside the housing for the HST is attached to the outside of the housing for the HST. Therefore, the second housing for the sun-and-planetary gearing, projecting outward from the housing for the HST, makes an open space therebelow so as to ensure a sufficient height for raising the mower disposed below the vehicle.
These, other and further objects, features and advantages will appear more fully from the following description.